How accurate is the 340-day estimate?

The 340-day average has a standard deviation of about 10 days. Most mares foal between day 330 and day 350. The full normal range is 320–360 days. Maiden mares tend to foal a few days later than experienced broodmares.

Does breed affect gestation length?

Slightly. Thoroughbreds average 338 days; draft breeds may average 342–345 days. However, individual variation within a breed is much larger than breed differences. This tool uses the 340-day consensus average.

When should I do a pregnancy check?

Ultrasound at 14–16 days post-breeding to confirm pregnancy and check for twins. A second check at 25–30 days confirms viability. Your veterinarian may recommend additional checks.

What if my mare goes past 360 days?

Gestation beyond 360 days is uncommon but can occur, especially in maiden mares. Consult your veterinarian — they may recommend monitoring or intervention depending on the mare’s condition.

Should I change the mare’s diet during gestation?

Nutritional needs are relatively stable through the first two trimesters. In the third trimester, energy needs increase 10–15%, and protein/mineral requirements rise. Consult an equine nutritionist for a pregnancy-specific feeding plan.

ToolsConductScience tool

340-Day AverageFree in-browser calculator

Mare Gestation Foaling Planner.

Enter a breeding date to calculate the expected foaling window, trimester stage, and days remaining.

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Breeding Information

Breeding / cover date

Gestation Timeline

Breeding date is more than 374 days ago — foaling should have occurred.

Enter a breeding date above to see the gestation timeline.

Based on Brinsko et al., "Manual of Equine Reproduction" (3rd ed.) and AAEP mare reproductive care guidelines. Normal gestation ranges from 320 to 360 days. Always work with your veterinarian for prenatal care scheduling.

How It Works

Equine gestation averages 340 days (approximately 11 months), with a normal range of 320 to 360 days. Due date equals breeding date plus 340 days. Most mares foal between day 320 and day 360; foals born before day 320 are premature and may need veterinary intervention. Trimesters: First (days 0–113) covers embryo fixation and pregnancy check at 14–16 days. Second (days 114–226) is when fetal growth accelerates and nutrition needs increase. Third (days 227–340+) involves rapid fetal growth, udder development, and foaling preparation. Maiden mares and older mares may gestate longer, and individual variation is normal.

Foaling Preparation Checklist

60 days before due date: give rhinopneumonitis (EHV) booster if not given at months 5, 7, 9, begin increasing feed by 10–15% for third-trimester energy needs, and set up foaling stall (12×12 minimum) with clean bedding. 30 days before due date: administer core vaccines (tetanus, EEE/WEE, West Nile, rabies if not current), perform Caslick’s removal if applicable, and assemble foaling kit with towels, iodine for navel, enema, and flashlight. 14 days before due date: begin nightly foaling watch, wrap tail, and monitor udder development and waxing. Signs of imminent foaling include waxing of teats (1–2 days before), relaxation of tailhead ligaments, vulvar elongation and relaxation, and restlessness, sweating, lying down and getting up.

Method

Due date = breeding date + 340 days (mean equine gestation). Early window = breeding date + 320 days. Late window = breeding date + 360 days. Trimesters are divided into roughly equal 113-day periods. Days remaining counts from today to the due date. Based on Brinsko et al., "Manual of Equine Reproduction" (3rd ed.) and AAEP reproductive care guidelines. All computation runs locally in your browser — no data is uploaded.

Validated

Last validated 2026-04-08. Calculations are designed for planning and documentation support; verify procurement decisions against manufacturer specifications or institutional SOPs.

How to cite

How to Cite

ConductScience Team (2026). Mare Gestation & Foaling Planner [Web application]. ConductScience. https://conductscience.com/tools/mare-gestation-foaling-planner

Equine Reproductive Biology

Mares are seasonally polyestrous, cycling during long-day months (April–September in the Northern Hemisphere). Gestation averages 340 days, the longest of common domestic species. The placenta is diffuse (epitheliochorial), meaning nutrient transfer is relatively inefficient compared to hemochorial placentas — which partly explains the long gestation. Fetal growth accelerates dramatically in the third trimester, when the foal gains 60–65% of its birth weight. This is why late-gestation nutrition is critical.

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